Evaluation of plasma polymer-coated contact lenses by electrochemical impedance spectroscopy.

The influence of ultra-thin (i.e., 5-50 nm) plasma polymer coatings on siloxane-based hydrogel contact lenses was investigated by electrochemical impedance spectroscopy (EIS). Impedance measurements as a function of frequency (Bode plots) were taken at regular intervals until steady impedance was obtained, indicating that the lenses were saturated. Appropriate equivalent circuit models were constructed to describe the salt intrusion characteristics of the plasma polymer-coated contact lenses. This provided information pertaining to the resistance and capacitance of interfacial and bulk layers in the plasma polymer-coated lenses. Resistance relates to ion permeability and capacitance for water uptake. This investigation showed that some of an ultra-thin layer of plasma polymer applied onto a dry hydrogel remains a contiguous film after the substrate hydrogel swells upon hydration. In some cases, however, the overall impedance of the coated lens is so low that the state of the plasma polymer layer after the hydration of the substrate hydrogel could not be judged by EIS.

Behavior of Model Proteins, Pretreated in Urea and/or Dithiothreitol, at Air/Solution Interfaces

The behavior of the model proteins, lysozyme, myoglobin, and beta-casein, pretreated in urea and/or dithiothreitol, at air/solution interfaces was studied by surface pressure-area techniques. The data suggested that in the absence of pretreatments the globular proteins are only partially unfolded at the interfaces. The interfacial activity was enhanced by the pretreatment (lysozyme in 8 M urea with 0.2 M dithiothreitol and myoglobin in 8 M urea). The interfacial activity of casein, a random-coil type protein, was not influenced by the pretreatment (8 M urea), as it readily and completely unfolds at the interfaces. The unfolding of globular proteins at the interfaces is apparently restricted by both disulfide and noncovalent bonds. Pretreatment can relax those restrictions, resulting in more complete interfacial unfolding. Copyright 1998 Academic Press. Copyright 1998Academic Press

Oxygen permeability of a new type of high Dk soft contact lens material.

Lotrafilcon A is a biphasic block-copolymer, comprising a highly permeable siloxane-based polymeric phase, coupled with a water phase (hydrogel phase). The high oxygen permeability of this material, and the fact that it is a hydrogel, places it outside the applicability of both the polarographic ISO standard and coulometric ISO draft standards for contact lens Dk determination. The oxygen permeability (Dk) and transmissibility (Dk/t) of lotrafilcon A lenses were determined by an adaptation of the standard coulometric method. Lenses with a thickness (t) range from 30 microns to over 300 microns were measured in a liquid-to-gas and a gas-to-gas configuration in an effort to combine features of the ISO (draft) standards to yield a valid measurement of the intrinsic material's oxygen transmission characteristics. The following results, for lotrafilcon A, are the mean values and SE for the oxygen permeability coefficient (Dk) determined at 34 degrees C: 140 +/- 2 barrer with water overlay (liquid-to-gas) at 2100 rpm stirring speed, 150 +/- 5 barrer with water overlay (liquid-to-gas) at "infinite" stirring speed, and 170 +/- 2 barrer without water overlay (gas-to-gas), where barrer = 10(-11) (mlO2.cm)/(sec.cm2.mm Hg). Clinical lenses produced from this material are expected to be in the 60 to 90 microns thickness range, with an average center thickness of 80 microns. Given that the Dkmaterial is greater than or equal to 140 barrers, for this material, a parallel sided lens of 80 microns central thickness would exhibit a lens transmissibility (Dk/t) of at least 170 x 10(-9) mlO2/(sec.cm2.mm Hg) at 34 degrees C. This transmissibility is well in excess of the 87 x 10(-9) mlO2/(sec.cm2.mm Hg) value postulated to be sufficient to prevent overnight lens-induced corneal swelling and places it in the hyper-permeable material category.

The Dk project: an interlaboratory comparison of Dk/L measurements.

The oxygen transmissibilities (Dk/L) of a set of 48 contact lenses made from 8 different materials were measured by 4 laboratories. The L/Dk measurements from each laboratory were compared and correlated. Samples which were not masked with a fixed front surface aperture during measurement were corrected for edge effects. This paper shows that provided L/Dk is calculated for each lens using the same technique and Dk is derived using a graphical method of calculation, similar results can be obtained by all laboratories. However, the agreement was less good for materials of Dk greater than 70 x 10(-11) (cm2/s) (ml O2/ml x mm Hg).